Distillation columns have, of course, been widely used to perform separations of all types in different industrial applications. Over fifty years ago, it was proposed to replace two distinct distillation columns with a single distillation column having a vertical partition (dividing wall column) within the column. This single, divided wall column could effect the separation of the column feed into three constituent fractions. It was recognized then that a dividing wall column could reduce, or minimize, the size or cost of the equipment needed to produce overhead, bottoms, and sidedraw products.
Control systems for dividing wall distillation columns have been known since at least 1980, when U.S. Pat. No. 4,230,533 described a dividing wall distillation column and its control system. In the late 1990's, control systems for dividing wall distillation columns have been studied in further detail by researchers.
Despite the advantages of the dividing wall column and despite much research and study, the processing industry has long felt reluctant to use dividing wall columns in commercial processes. This widespread reluctance has been attributed to various concerns, including control problems, operational problems, complexity, simulation difficulties, and lack of design experience. General guidelines and considerations when substituting a dividing wall column for conventional columns have been established. Nevertheless, there are relatively few documented practical uses of dividing wall columns in commercial plants.
It is known, for example, to provide a partition in such a column to thereby subdivide the interior of the column into an inlet region and a discharge region. The partition in this system serves to prevent transverse mixing of liquid and/or vapor streams over a limited height of the column. The apparatus reduces the number of distillation columns required for the separation of multicomponent feed stocks.
The partitions can extend between opposite walls of the column and can have heights which, as a rule, can be greater than the column diameter and can be at least equal to the spacing between individual horizontal bottoms of the column. The use of continuous partitions has, however, a number of drawbacks and these drawbacks are the more serious the greater the diameter of the column.
The incorporation of a one-piece partition in the column is difficult because of its weight and the need for a close fit of the partition in the column. From a certain size, the dimensions of such partitions ensures that they can only be handled with mechanical assistance which can lead to additional costs. Furthermore, with increasing weight, the partitions can pose a greater danger for the workers who are necessary for installation of the partition. It has been found in practice that the partition can easily become canted during installation in the column and can thereby damage the inner wall of the column or result in poor sealing between the column and the partition. Irregularities in the shape of the column can also limit the sealing effectiveness. Since possible nonsealing regions may then have to be sealed up by additional acts, the costs of the column and the partition can be prohibitive.